A method for forming a film on a substrate such as a semiconductor wafer or the like (hereinafter, referred to as “wafer”) includes a CVD (Chemical Vapor Deposition) method for forming a film by supplying a source gas to a surface of a wafer and reacting the source gas by, e.g., heating the wafer, an ALD (Atomic Layer Deposition) method for allowing an atom layer or a molecular layer of a source gas to be adsorbed on a surface of a wafer, generating a reaction product by supplying a reactant gas that oxidizes and reduces the source gas, and depositing a layer of the reaction product by repeating such processes, and the like. These processes are performed by supplying a source gas into a reaction chamber (processing chamber) in which a wafer is accommodated and a vacuum atmosphere is present.
Most raw materials used in the CVD, the ALD or the like often have low vapor pressures. In that case, the source gas is obtained by supplying a carrier gas to the raw material tank accommodating a liquid or solid raw material and vaporizing the raw material in the carrier gas. Meanwhile, in order to control a thickness or a film quality of a film formed on a wafer, the amount of the raw material contained in the source gas needs to be accurately controlled.
For example, Japanese Patent Application Publication No. 2006-52424 (paragraph 0067 and FIG. 15) discloses a technique for monitoring a flow rate of a vaporized raw material by deducting a flow rate set value of a carrier gas from a measured flow rate of a source gas that is a gaseous mixture of the vaporized raw material and the carrier gas in case the source gas generated by supplying the carrier gas to a raw material tank.
However, in the above-described method, even if the flow rate of the carrier gas is controlled at a constant level, the flow rate of the vaporized raw material may change due to the decrease in the amount of the raw material in the raw material tank and the changes in residence time of the carrier gas, for example. In this regard, Japanese Patent Application Publication No. 2006-52424 does not disclose how to control a flow rate set value of the carrier gas for obtaining a source gas containing a desired amount of the raw material. Accordingly, even if the flow rate of the vaporized raw material is monitored, the flow rate set value of the carrier gas is adjusted by trial and error, and a long period of time is required until the accurate flow rate of the vaporized raw material is obtained.